ISCET Semiconductor Devices 2 — Questions and Answers
Question 1: What is the primary function of the depletion region in a reverse-biased PN junction?
- To increase forward current
- To act as an insulating barrier that blocks majority carrier flow (Correct answer)
- To reduce junction capacitance to zero
- To enhance minority carrier recombination
Correct answer: To act as an insulating barrier that blocks majority carrier flow
The depletion region widens under reverse bias, creating an effective insulating barrier that prevents majority carriers from crossing the junction.
Question 2: A Zener diode is operating in reverse breakdown at 6.2 V with 20 mA flowing through it. What is the power dissipated?
- 0.31 W
- 124 W
- 0.124 W (Correct answer)
- 3.1 W
Correct answer: 0.124 W
Power = V × I = 6.2 V × 0.020 A = 0.124 W.
Question 3: Which BJT configuration provides the highest voltage gain but has a current gain less than 1?
- Common emitter
- Common base (Correct answer)
- Common collector
- Darlington pair
Correct answer: Common base
The common-base configuration has a current gain (alpha) slightly less than 1 but can provide significant voltage gain.
Question 4: In an N-channel JFET, pinch-off voltage (Vp) is the gate-to-source voltage at which:
- Drain current reaches its maximum value
- The channel is fully depleted and drain current drops to nearly zero (Correct answer)
- The device enters saturation
- Gate current begins to flow
Correct answer: The channel is fully depleted and drain current drops to nearly zero
Pinch-off voltage is the negative VGS value that fully depletes the N-channel, reducing drain current (IDSS) to essentially zero.
Question 5: Which parameter describes the ratio of the change in collector current to the change in base current in a BJT?
- Alpha (α)
- Beta (β) (Correct answer)
- Gamma (γ)
- Transconductance (gm)
Correct answer: Beta (β)
Beta (β or hFE) is defined as IC/IB and represents the DC current gain of a BJT in common-emitter configuration.
Question 6: A silicon diode has a forward voltage drop of approximately 0.7 V. If a germanium diode is used instead, the forward voltage drop would be approximately:
- 1.4 V
- 0.7 V
- 0.3 V (Correct answer)
- 0.1 V
Correct answer: 0.3 V
Germanium diodes have a lower barrier potential of approximately 0.3 V compared to silicon's 0.7 V.
Question 7: What is the effect of increasing temperature on the reverse saturation current (IS) of a PN junction diode?
- IS decreases exponentially
- IS remains constant
- IS increases, roughly doubling for every 10°C rise (Correct answer)
- IS becomes negative
Correct answer: IS increases, roughly doubling for every 10°C rise
Reverse saturation current approximately doubles for every 10°C temperature increase due to increased thermally generated minority carriers.
ISCET Semiconductor Devices 2
What is the primary function of the depletion region in a reverse-biased PN junction?